Resonant free piston Stirling engine systems are known in the art wherein the load apparatus is hydraulically driven from the periodic pressure wave of the engine. In such known systems the load apparatus is typically disposed within an incompressible fluid-filled space between a pair of flexible diaphragms which seal in and isolate the incompressible fluid, referred to herein as “hydraulic fluid”, from the Stirling Engine. One of the diaphragms is arranged to be acted on by the resulting pressure wave produced in the hydraulic oil and the other diaphragm is arranged as part of a gas spring. The pressure waves produced in the hydraulic oil are operative to reciprocally drive the movable member of the load apparatus in a direction along the same axis as that of the Stirling Engine. Such prior art engine-driven system assemblies were arranged in a stacked, coaxial relationship. While generally satisfactory, the diaphragms employed dramatically limited the useful life of such a device before maintenance was required. Other prior art arrangements had the load components immersed in the hydraulic oil making maintenance, service and repair difficult and expensive.